In a first known application, BRDF measurements can be used to characterize the signature of an aircraft.
For several years, BRDF measurements have been used in software for synthesizing images, for computer-assisted design (CAD), and for simulating light in order to simulate the behavior of surfaces in response to light.
For this purpose, such software makes use, e.g. in the form of a library, of real BRDF measurements performed in a laboratory on the various types of object or surface presenting optical behavior that is it desired to simulate.
In order to perform such BRDF measurements, the best known apparatuses make use of a sample of the object to be characterized. The sample must be of small dimensions, typically of the order of 10 centimeters (cm).
Such measurement apparatuses generally comprise articulated arms of large dimensions having detectors placed at their ends. Consequently they are bulky, heavy, and in practice difficult to transport.
Furthermore, the time required to acquire measurements with such apparatuses is very long, in particular because of the movements of the sensors.
Consequently, they are used exclusively in measurement laboratories and they are not adapted to taking measurements on site.
An object of the invention is to obtain a device for measuring BRDF that is of reasonable size, transportable, and thus suitable for use on site, and that enables measurements to be performed almost instantaneously, by having an acquisition time that is very short.
U.S. Pat. No. 5,637,873 (Davis) describes a BRDF measuring device suitable for use on site, e.g. to measure certain optical properties of the surface of a vehicle.
With reference to its FIG. 5, the Davis patent describes an embodiment of that device comprising a system for collecting the light reflected by an object that is to be characterized, which system is constituted by two lenses and an elliptical reflector.
More precisely, in that embodiment, the sample of the object to be characterized is placed at a first focus of the elliptical reflector, with the light reflected by the sample being focused on the second focus of the elliptical reflector.
A first lens placed at said second focus is arranged in such a manner as:                to focus the light rays that are reflected by the sample and that are not reflected by the elliptical reflector; and        to avoid deflecting the light rays that are reflected by the elliptical reflector.        
The Davis system uses a second lens downstream from the first lens, said second lens being remarkable in that it has both a first optical portion for collimating the light rays collected by the first lens, and a second optical portion for collimating the light rays collected by the elliptical reflector. That specific second lens thus presents a discontinuity at the junction between the two above-mentioned optical portions.
That discontinuity requires the elliptical reflector and the two above-mentioned lenses to be accurately aligned, since otherwise rays reflected by the elliptical reflector become mixed in with rays focused by the first lens, and vice versa. That problem is known to the person skilled in the art under the term “cross-talk”.
Japanese patent document JP 63-140904 A (Toshiba) describes a compact device for measuring the light intensity of an object, that device has a condenser lens and a reflective tubular body, with a surface array sensor being placed at the outlet thereof.
Although not described explicitly, the person skilled in the art will understand that a physical embodiment of the device requires an array of sensors of large size (typically with a diagonal of more than 50 millimeters (mm)) in order to collect all of the light diffused by the object.
The Toshiba measurement device therefore cannot use presently known standard sensors (having a diagonal of about 12.7 mm), so it is necessarily expensive to manufacture.
One solution for solving that problem is to add an afocal system between the reflective tubular body and the sensor array. That enables the outlet beam section to be reduced while conserving light collimation. An arrangement of that type is indeed used in the Davis system where it enables a small-sized video sensor to be used to observe a beam of large diameter.
However, the major drawback of that solution is that the observation plane is moved away from the reflective tubular body. Since beam divergence is never exactly zero, that leads to a corresponding degradation in resolution in the observation plane, particularly when the sample for measurement is illuminated over a large area. Consequently, the Toshiba device does not enable accurate angular measurements to be made in the intensity pattern of the diffused light.